In recent years, carbon fiber composite materials have been used in a wide field of applications including sports, aerospaces and industries and the consumption thereof is remarkably increasing in quantity. In correspondence to such conditions, the properties of carbon fibers used are also being improved by leaps and bounds.
In regard to the modulus of elasticity of carbon fibers, whereas it was about 20 ton/mm.sup.2 ten years ago, 23-24 ton/mm.sup.2 became its standard value several years ago. Further, recent efforts in development are being directed to attaining a modulus of elasticity of about 30 ton/mm.sup.2, and it is generally believed that such a value would become the mainstream of the moduli of elasticity of carbon fibers.
However, if such improvement of the modulus of elasticity of a carbon fiber is achieved while keeping the tenacity of the carbon fiber at a constant value, it will naturally cause the decrease of elongation of the carbon fiber, which will result in brittleness of carbon fiber composite materials produced by using such carbon fibers and in lowering the reliability of the properties of the composite materials.
Accordingly, there is a strong need at present for a carbon fiber having a high modulus of elasticity and a high elongation, in other words, a carbon fiber having a characteristic that it has a high elongation and at the same time has a high tenacity.
Conventional methods for improving the modulus of elasticity of a carbon fiber comprised increasing the carbonization temperature, namely the ultimate heat-treatment temperature, of the carbon fiber. However, though such a method is effective in improving the modulus of elasticity of carbon fibers, it has a defect in that the improvement is accompanied by the decrease in the tenacity of the carbon fibers and consequently results in the decrease in the elongation of the fibers. The attached drawing is a graph showing the correlation between the carbonization temperature of a carbon fiber and the physical properties of the resulting carbon fiber to illustrate such situations. As shown in the drawing, with the increase of the carbonization temperature of a carbon fiber, the modulus of elasticity of the fiber increases as indicated by curve A, whereas the tenacity and the density of the carbon fiber decrease as shown by curves B and C in the drawing in keeping with the above increase of the modulus.
For example, a temperature of about 1800.degree. C. is fibers obtained according to the above invention has a tenacity of 360 to 420 kg/mm.sup.2 and a modulus of elasticity of fiber strand of high tenacity and high modulus of elasticity.